1. Department of Biochemistry and Biophysics, 1210 Molecular Biology Bldg., Iowa State University, Ames, Iowa 50011, Department of Molecular Biology, St. Petersburg Nuclear Physics Institute, Gatchina, St. Petersburg district, 188350, Russia, and Department of Chemistry, Carlsberg Laboratory, Gamle Carlsberg Vej 10, DK-2500 Copenhagen Valby, Denmark